Styrene resins are advantageous in that economic efficiency is excellent due to low-priced monomers used to prepare the resins and the transparency is excellent. Films which are produced by using the styrene resins are materials having the optical isotropy increasing the refractive index in a direction perpendicular to the alignment direction when the films are stretched and aligned, and considered materials useful to prepare a film having a positive thickness retardation (Rth) value by using the stretching.
However, the film that is manufactured by using the styrene resins are disadvantageous in that the heat resistance and the mechanical properties are poor, with the exception of when the resins are prepared in conjunction with costly special monomers. If the resins are blended with amorphous polyester blocks in order to avoid the disadvantages of the styrene resin, performance of the resulting composition is reduced due to low compatibility.
Meanwhile, in respects to the production of retardation films, Japanese Unexamined Patent Application Publication Nos. 2000-162436 and 2000-304925 disclose a process of producing a retardation film, which includes attaching an inverse shrinkable film to a film of a thermoplastic resin to perform stretching. However, in the process, it is difficult to control the refractive index in order to increase the z-axis direction refractive index.
Korean Registered Patent No. 0484085 discloses a process of providing a z-axis direction refractive index by using a combined substance of optical devices including one or more optical retardation films and one or more broadband reflective polarizers. However, this patent requires a complicated process.
Korean Unexamined Patent Application Publication No. 2004-29251 discloses a process of providing a z-axis direction refractive index, which includes forming a film by using a copolymer obtained by copolymerizing olefin and N-phenyl substituted maleimide and uniaxially or biaxially stretching the film. However, the patent is disadvantageous in that since material having a high glass transition temperature is used, a stretching process is performed at a high temperature of 220° C. or more and the thickness retardation (Rth) of the film having a thickness of 100 μm after +50% stretching is undesirably controlled to be 100 nm or less.